Electrical resistance heating element with a honeycomb body

ABSTRACT

The aim of the invention is the application of simple production methods for electrical resistance heating elements with a honeycomb body and use total capacity with regard to the specific electrical power per volume unit. Said aim is achieved for an electrical resistance heating element with a honeycomb body of resistance material with positive temperature coefficients for the resistance (PTC resistance), in which the channels, for throughflow of a medium for heating, comprise metallised walls and are electrically alternately contacted after the fashion of a chess-board on the front faces of the honeycomb body by means of a front face metallisation, whereby an insulation region ( 1 ) is alternately let into the metallisation ( 3 ) on the walls of the channels running from the front faces, whilst the front face metallisation ( 2 ) completely covers the front faces. Possible fields of application for the above (without limitation thereto) are particularly additional heaters and windscreen heaters in motor vehicles or small, self-regulating and rapidly-responding room space heaters.

The invention relates to an electrical resistance heating element with ahoneycomb body made of resistance material with positive temperaturecoefficients (PTC resistance). Such heating elements use the possibleinternal contact of a fluid conducted through the channels of thehoneycomb body with the site at which the heat occurs, whereby good heattransmission is achieved, and also use the current-limiting effect ofthe PTC resistances, which automatically prevents any overheating of theresistance heating element without any additional regulating elements.When the operating current can be conducted through the walls of thehoneycomb body channels quasi-transversely to the direction of flow ofthe fluid, the applicable operating voltage interval clearly increasesand essentially depends on the specific material parameters. There is agreater degree of freedom in dimensioning, since the realizable channellength is very limited with current flow in the channel direction, evenfor a power supply.

One technical solution that has become known that is not based on ahoneycomb body but that approaches it in terms of the heat exchangeprinciple and that also contains PTC resistances is comprised in thatrectangular PTC bodies are alternately coated with meandering corrugatedmetal strips and compressed. The metal strips supply the current for thePTC body and also act as heat exchangers in that a fluid flows throughthem transverse to the flat stack (Company publication, “PTC coldconductor/heat register, Type HR 02”, David+Bader GmbH, Kandel). Thisarrangement suffers from the two following principal deficiencies. Thenon-positive electrical contacting is technologically complex and canlead to irregular temperature distribution due to transition resistancesdespite the self-regulating effect of PTC resistances described in theforegoing. The same applies to the heat stream in the direction from thePTC body to the metal strips, whereby it also becomes immediatelyapparent that the heat exchange occurs indirectly at the corrugatedmetal strips, not directly at the site at which the heat occurs.

Furthermore known is a honeycomb body for electrical heating in whichthe corrugated layers of a typical semiconducting ceramic alternate withlayers made of metal or materials containing graphite that are intendedto act as current feeds (Patent Abstract of Japan no. 02 129 887 A).Apart from the fact that the preferred embodiment in this publicationcites as resistance materials silicon and titanium carbides andsuicides, whose electrical resistance decreases as voltage increases sothat they lack the aforesaid self-limiting effect, such a surface fitcombination of different materials is difficult to produce and thedurability of its individual components can suffer given different heatexpansion behaviors.

A PTC honeycomb body with channels interiorly metallized on all sidesfor application in an automobile heater system is also known, wherebycontacting is indicated, first, by metallizing the exterior wall of thehoneycomb body, and second, by connecting wires that extend into theinitial area of each channel (U.S. Pat. No. 4,866,365). When the channeldimensions are small, contacting using connecting wires is veryproblematic in terms of production engineering and also interferes withthe flow ratio in the channels in an adverse manner.

Furthermore known is a PTC honeycomb body with channels interiorlymetallized on all sides for heating gas flowing therethrough in whichthe electrical contacting of the metallized channel walls occurs via endface-side slits that are in the channel walls and that are alsometallized, whereby these channels are interconnected in a diagonaldirection in order to create the required checkerboard polarity (DE 3016 725 A1). This technical solution is also disadvantageous forproduction engineering reasons and due to the flow ratios. In addition,the stability of the honeycomb body might also be adversely impacted dueto the notch effect of the slits.

Finally, resistance heating elements of the aforesaid type are known inwhich the contacting occurs exclusively through a contacting pattern oranother surface-like contacting arrangement on continuous flat endfaces, specifically for direct current corresponding to the knowncheckerboard arrangement (DE 198 04 496 A1) and for rotary current bymeans of very special metallizing patterns, preferably but notexclusively in channels with a triangular or hexagonal cross-section(U.S. Pat. No. 4,717,813 A). These arrangements have the followingdisadvantages:

-   -   The width of the strip conductors on the end faces is limited        geometrically by the wall thickness of the honeycomb body,        whereby in addition the required insulation distance to the        channels with electrically opposite poles must also be        maintained.    -   The interior contacting of the channels in principle makes        possible higher specific electrical power of such resistance        heating elements, whereby however necessarily higher currents        are required in the contacting patterns so that the highest        current densities occur there based on design.    -   The maximum permissible current load of the contacting pattern        leads to a limit of the power spectrum, that would otherwise be        possible in principal, of the resistance heating elements with        honeycomb body.    -   The production methods for applying the filigree strip        conductors of the contacting pattern are very complex, and, due        to the production tolerances and the application methods for the        strip conductors and due also to the thickness of the channel        walls of the honeycomb body, these must be embodied even        narrower than would actually be necessary for electrically        insulating the opposite poles.

The object of the invention is to make possible simpler productionmethods for electrical resistance heating elements of the type specifiedin the foregoing and to exhaust the reserves of such resistance heatingelements in accordance with the prior art with respect to the specificelectrical power per unit of volume.

This object is achieved by the invention described in the patent claims.

The invention achieves the following advantages:

-   -   The metallization of the channel walls is conducted just on one        side to the end face and is contacted there. On the other side,        the metallization terminates a specific distance before the end        face, resulting in insulation. 1 mm is an adequate distance for        extra-low voltage.    -   The contacting and non-contacting of the metallization of the        walls of the channels alternate in a checkerboard pattern, so        that the operating voltage is applied between two adjacent        channels.    -   The inventively continuous metallization of the end faces leads        to substantially higher current-carrying capacity and thus        permits the required exhaustion of the power spectrum of this        type of electrical resistance heating element.    -   The inventively continuous metallization of the end faces is        much simpler in terms of production engineering than applying        fine conductor strip structures, as is provided in the invention        in the most closely related prior art.

Interesting areas of application for the invention result from heatingmotor vehicles, without being restricted thereto, such as for instance

-   -   supplementary heating for motor vehicles, when the heat from        modern economic combustion engines is not adequate for heating;    -   rapid front windshield heating when the engine is cold;    -   sole heating in electrical vehicles.

Additional application options, which likewise are not restrictive,result for small, self-regulating, and rapidly responding space heaters.

The invention is described in the following using an exemplaryembodiment. The attached drawing is a schematic perspective view of aninventive resistance heating element.

Regardless of the technical embodiment of the invention-essentialcontacting, a suitable honeycomb body material and its manufacture willfirst be described as an example. Such a material is a ceramic materialthat correspond to the following total chemical formula:La_(0.0028)Ca_(0.22)Ba_(0.7772)Ti_(1.015)Si_(0.02)Mn_(0.0006)

The initial materials (generally compounds that can be broken down intooxides, such as carbonates, acetates, etc.) are mixed intensively andthen carefully annealed for forming the oxides. Grinding follows, andfinally, with conventional plasticizers, it is extruded into honeycombbodies followed by ceramic firing.

The finished ceramic has the following electrical properties: Specificresistance at 20° C.: 50 Ωcm R_(max)/R_(min): 2.2 × 10⁵ Tc: 100° C.T_(max): 232° C. Electric strength >150 V/mm

The attached drawing speaks largely for itself. The schematicallyrepresented honeycomb body with rectangular channel cross-sections hasgray-shadowed metallizations, namely continuous on the end faces (ofcourse, only one of these is visible (it is labeled 2)) and on the wallsof the channels (labeled 3). The blank insulation areas 1 are essentialto the invention; it should be noted that of course it should be assuredthat there is a good electrical connection between the metallizations 2and 3 at sites where no insulating area 1 is provided, as in the channellabeled 3.

1. (canceled)
 2. Electrical heating resistance element comprising ahoneycomb body comprised of electrical resistance material havingpositive temperature coefficients of resistance, the honeycomb bodybeing comprised of channels through which medium to be heated is to flowand the honeycomb body having end faces and metallized channel walls,the end faces being completely covered by metallization, the metallizedwalls of a first set of the channels forming a first checkerboardpattern electrically connected by the metallization of only one of theend faces and insulated from the other of the end faces, the metallizedwalls of a second set of the channels forming a second checkerboardpattern complementary to the first checkerboard pattern, and the wallsof the second set of channels being insulated from the metallization ofsaid one of the end faces and electrically connected to themetallization of said other of the end faces.